The tread portion of a pneumatic tire generally comprises a plurality of circumferentially and laterally extending grooves defining ground engaging rubber elements, the elements being in the form of blocks or ribs or combinations thereof. The particular size and shape of the tread elements contribute significantly to the overall performance of the tire and are for that reason designed to achieve the desired tire characteristics.
Winter type tires, as well as all-season type tires, usually have multiple sipes. A sipe is a groove having a width in the range of about 0.1% to about 1% of the tread width, i.e. the arc length of the tread surface in the axial direction. The sipe tends to close when it is located in the tire footprint at zero speed and under normal load and pressure. Sipes are typically formed by steel blades inserted into a cast or machined mold or tread ring therefor.
A sipe may extend circumferentially or laterally about the tread in a straight, curved, or zigzag manner and may be as deep as the primary tread grooves or have a depth that is greater than the groove depth. The sipes can pass through the sides of the ribs and tread blocks or be confined to the interior of the tread elements. It is also known to have sipes lying in planes which are not perpendicular to tangents to the surface of the tread at their point of intersection; the inclination of the planes defining neighboring sipes can be identical or differ step wise along the tread element length. It is further known to use sipes having a depth that varies along its length.
The presence of sipes in a tread increases the number of biting edges in the tread. The local high pressure at each biting edge improves the wiping and digging action of the tread surface, conferring to a tire excellent traction on snow and ice. Furthermore, sipes improve the flexibility of the tread elements without destroying their solidity. The easy relative longitudinal sliding between the opposed faces of the sipe weakens the resistance of the tread elements to flexing in the contact area between tread and road and therefor slows down the heat buildup of the tire; however, the sliding of the opposed faces of the sipes creates friction between the opposing sipe faces and can lead to wear of the sipes.
GB 1,150,295 discloses a tire with a rib having a plurality of sipes formed therein. The sipes have a three-dimension aspect, wherein a plurality of peaks and valleys are defined though a sipe centerline. The peaks and valleys formed are either a single point of maximum depth or a line of maximum depth. Due to the sharp lines forming the sipe configuration, while the opposing sides of the sipe interlock, there is increased abrasion at the points.
U.S. Pat. No.5,783,002 discloses a three-dimensional sipe for tire treads. The protrusions and cavities formed all have a domed configuration. The rounded opposing edges of the sipe reduces rubbing but doesn't eliminate the movement of the opposing sipe faces. EP 1073562 shows circular 3-d siping with flat spaces therebetween. The flat portions of the sipe are still subject to rubbing and wear.